White light led with multicolor light-emitting layers of macroscopic structure widths, arranged on a light diffusing glass

ABSTRACT

Multicolor-emitting, for example organic light emitter layers ( 1, 2, 3 ), the emission spectra of which can be mixed to form white light, are arranged laterally next to one another on a first transparent electrode layer ( 7 ). The light emitter layers ( 1, 2, 3 ) are preferably in strip form and have, for the purpose of simplifying production, macroscopic dimensions that can be resolved by the eye. The thickness of the light-scattering substrate ( 4 ) is therefore chosen such that the radiation beams emitted by the light emitter layers ( 1, 2, 3 ) are at least partly superposed on the light exit surface of the substrate ( 4 ), thereby giving rise to the optical impression of a white light source. (FIG.  1  for publication with the abstract)

The present invention is generally concerned with the field ofelectroluminescent light emission devices and relates, in particular, toan electroluminescent light emission device according to the preamble ofpatent claim 1, which can be used as a white light source.

In the past, various concepts have been developed in the endeavor toreplace conventional white light sources based on incandescent orhalogen lamps.

The furthest advanced concept is based on a semi-conductor LED based onGaN or InGaN, which is at least partly embedded in a transparent castingresin material containing a converter substance for an at least partialwavelength conversion of the light emitted by the LED. In this case, theLED preferably has a plurality of light-emitting zones which generate arelatively broadband light emission spectrum above the emission spectrumof the converter substance in energy terms. The light in the blue or UVwavelength range emitted by the GaN-LED is at least partly converted bythe converter substance into light having a wavelength in the yellowspectral range upon passing through the casting resin composition, sothat white light is generated by additive color mixing. On the basis ofthis concept, small-format luminous sources such as torches and the likehave recently been introduced commercially. However, these productsstill have various disadvantages. First of all, the spectral lightemission curve of these white light sources is still not optimal, sothat the physiological-optical impression of a white light source is inmany cases not provided—at least not from all viewing angles.Furthermore, the luminous intensity of the white light sources which canbe produced according to this concept is limited on account of the stillexcessively low light power of the GaN-LEDs and the necessary lightconversion. Therefore, large-area white light sources having a highluminous intensity are not expected in the near future on the basis ofthis concept.

A further concept for producing white light sources is based on themonolithic integration of a plurality of semiconductor layers andcorresponding pn junctions on a substrate, the semiconductor materialsbeing composed such that light having different wavelengths is generatedin the pn junctions through current injection. U.S. Pat. No. 6,163,038describes such a white light source based on the GaN material system.Two embodiments describe components in which a semiconductor layersequence with three pn junctions is produced, which emit light havingwavelengths of the complementary colors red, green and blue, therebybringing about the optical effect of a white light source. This concepthas the advantage that, in principle, polychromatic light of any desiredcolor can be generated, thus resulting in a multiplicity of applicationsin the area of display and illumination devices. However, even withcomponents of this type, it will not be possible to produce large-areaillumination devices having a high luminous intensity with a tenableoutlay in respect of costs.

A further concept for white light sources is based on organiclight-emitting diodes (OLED), which have developed rapidly in recenttimes and are already used commercially in display devices of motorvehicles. Organic light-emitting diodes can generate white lightrelatively simply, since, by virtue of the great diversity of organicsubstances, different emitters can be brought together in order togenerate white light. The published German patent application DE 199 16745 A1 describes a light-emitting diode with organic light-emittingsubstances for generating light with mixed colors, in which a layercontaining organic light-emitting substances is arranged between twoelectrode layers and two substrate carriers, in which case one of thecarriers may be formed by a diffusing screen. The organic light emitterlayer has a strip structure in which at least two types of strips arearranged alternately and strips of one type in each case having organiclight-emitting substances which emit light of a specific color, stripsof one type being driven jointly. The organic light-emitting substancesof the two or three strip types are chosen such that, given suitabledriving, the mixing of the radiated light of the strips produces whitelight. However, one disadvantage of the component described in theembodiment is that the width of the strips of the light emitter layersis so small that the strip structure can no longer be resolved uponnormal observation by the human eye, as a result of which the productionof the component becomes relatively complicated.

Accordingly, it is an object of the present invention to specify anelectroluminescent light emission device with which visible light ofvarying wavelength can be generated and additively mixed and which canbe produced cost-effectively. The electroluminescent light emissiondevice is intended to be able to be used, in particular, as a whitelight source.

This object is achieved by means of the characterizing features ofpatent claim 1. Advantageous developments and refinements are specifiedin the subclaims.

The present invention proceeds from a light emission device as has beendescribed in the document DE 199 16 745 already mentioned and in whichthe light emitter layers are arranged in the form of a strip structurealternately laterally next to one another between the electrode layers.

The present invention is based on the essential insight that it is notabsolutely necessary to provide a microscopic structuring of the stripstructure for the additive color mixing of the radiation beams emittedby the light emitter strips. Rather, it is an essential feature of thepresent invention that the lateral extent and/or the lateral distance ofthe light emitter layers from one another is of an order of magnitudesuch that it can be resolved by the human eye merely upon observation.For the color mixing of the radiation beams emitted by the light emitterlayers, it is then necessary that the thickness of the substrate formedas a diffusing screen is chosen such that the radiation beams emitted bythe light emitter layers are at least partly superposed on the lightexit surface of the substrate. Thus, the substrate must have a thicknessat least such that the individual radiation beams to be mixed experiencea sufficient scattering widening when passing through the substrate.

Consequently, there is no need for any complicated masking andstructuring steps, such as are known for instance from microelectronics,in order to produce an electroluminescent light emission deviceaccording to the invention. It is possible to provide macroscopiclateral extents of the light emitter layers, which can be produced bysimple structuring measures.

A standard value which is known from the resolution of the human eye is1 angular minute=1/60 degree. Given a viewing distance of 1 m from thelight emitter layers, this corresponds to 0.29 mm. If a viewing distanceof 30 cm is assumed, a resolution of approximately 100 μm results.Consequently, a value of approximately 100 μm for the lateral distanceand/or the width of strip-type light emitter layers can be assumed as anexpedient lower limit for lateral dimensions of the light emitter layersthat can still be resolved by the human eye.

With regard to the thickness of the substrate formed as a diffusingscreen, this should be greater, preferably significantly greater, thanthe average scattering length for the radiation beams emitted by thelight emitter layers in the diffusing screen. In a somewhat simplifieddefinition, the scattering length may be understood here to be theaverage free path length covered by a photon having a given frequencybefore it changes its direction as a result of scattering. A diffusingscreen having relatively strongly scattering properties thus has acorrespondingly short average scattering length, so that the thicknessof the diffusing screen can be dimensioned to be correspondingly smallerthan in the case of a diffusing screen having less strongly scatteringproperties. In order to attain a sufficient superposition of thedifferent radiation beams, the distance between the light emitter layersshould furthermore play a part for the thickness of the diffusingscreen. Preferably, in addition to the criterion mentioned above, thethickness of the diffusing screen should be chosen to be at leastapproximately as large as the distance between the light emitter layers.

In this case, the light emitter layers are preferably produced fromorganic materials in a manner known per se. As was shown experimentallyonly recently, organic materials suitable for this may be deposited insuch a way that charge carriers can enter into the organic layersthrough two electron- or hole-injecting electrodes and an organiclight-emitting diode (OLED) is thus formed.

Two or more different types of light emitter layers having differentemission spectra are provided, the mixing of which enables white lightto be generated. It may be provided that two unstructured electrodelayers extend on both sides of the light emitter layers, so that thelight emitter layers are always driven jointly. In this case, however itis not possible to take account of the fact that the different organicmaterials of the light emitter layers are subjected to different agingtimes, so that the optical impression of a white light source is lostover the course of time, since one of the light emitter layers loses itsluminosity more rapidly than the others.

Therefore, an advantageous embodiment provides for a plurality ofelectrodes to be arranged on one side of the light emitter layers insuch a way that light emitter layers with a different emission spectrumcan be driven separately from one another and those with an identicalemission spectrum can be driven jointly. Consequently, in use, thepossibly decreased luminosity of one type of light emitter layers can becorrespondingly compensated for by application of an increased voltage.Irrespective of this problem area of different aging times of theorganic materials, this embodiment can be used to control asindividually desired the coloration of an illumination device formed bythe white light source, i.e., by way of example, to emphasize the red orthe blue color component more strongly or more weakly within the whitelight spectrum.

In a simple embodiment which can be used for a small-format white lightLED, for example, it may be provided that the emission spectra providedfor the light mixing are represented only by one light emitter layer ineach case. However, embodiments are also conceivable, in particular forlarge-area illumination devices, in which the emission spectra arerepresented by a plurality of light emitter layers in each case. If thelight emitter layers are structured in strip form, then it is possible,in this case, for the light emitter layers to be arranged in analternate order according to their emission spectrum.

However it is not a necessary constituent part of the invention that thelight emitter layers are structured in strip form. Rather, the lightemitter layers may also be arranged as punctiform or circular-disk-typelayers laterally next to one another. Particularly if the light emitterlayers are formed by red, green and blue emitting light emitter layersin their emission spectra, the light emitter layers may be arranged inthe form of a color triad comprising three circular-point-type lightemitter layers, that is known per se from color display screentechnology. In the case of such an embodiment, with regard to thecriterion of resolvability by the human eye, a value of approximately100 μm may likewise be assumed as the lower limit for the diameter ofthe circular layers and/or for the mutual distance between their centerpoints.

An advantageous embodiment provides three types of light emitter layers,the essentially monochromatic emission spectra of which respectivelyhave maxima in the red, green and blue wavelength ranges. The mixing ofthe three complementary colors generates white light if they are emittedwith the same luminous intensity by the light emitter layers.

However, it may also be provided that only two types of light emitterlayers are used, the emission spectra of which respectively have maximain the yellow and blue wavelength ranges, it being necessary to set theyellow light emitter layer with a higher luminosity.

It is also conceivable to use more than three types of light emitterlayers having different emission spectra, for instance in order toenable the white light spectrum to be adapted as optimally as possibleto daylight.

An embodiment of the light emission device according to the invention isexplained in more detail below with reference to the figures, in which:

FIG. 1 shows a side or sectional view of a white light source accordingto the invention;

FIG. 2 shows a plan view of the white light source of FIG. 1.

The figures illustrate an embodiment comprising threevaricolored-emitting light emitter layers. The light emitter layers areformed by organic LEDs.

A first transparent electrode layer 7 is applied on a light-scatteringsubstrate 4, which may be formed for example by an opal glass screen ora plastic substrate with embedded scattering centers. The electrodelayer 7 may be formed for example by an ITO (indium tin oxide) layer.

Afterward, light emitter layers 1, 2 and 3 are successively applied instrip form to the first electrode layer 7. The light emitter layers 1, 2and 3 in each case comprise a layer sequence made of n- and p-dopedorganic materials, so that a pn junction is formed within the layersequence. The organic materials are chosen such that the first lightemitter layer 1 has an emission maximum at a wavelength in the redspectral range, the second light emitter layer 2 has an emission maximumat a wavelength in the green spectral range, and the third light emitterlayer 3 has an emission maximum at a wavelength in the blue spectralrange.

The strip-type light emitter layers 1, 2 and 3 can be perceived by thenaked eye on the substrate 4 and for example have a strip width of about1 mm.

The radiation beam emitted by the first light emitter layer 1 isillustrated by way of example. It is important for the color mixing andthe generation of white light that the average lateral scatteringwidening of the emitted light is significantly greater than the width ofthe strip-type light emitter layers. It is only if a sufficientsuperposition of the radiation beams emitted by the light emitter layers1, 2 and 3 occurs on the light exit surface of the substrate 4, oppositeto the light emitter layers, that the light emission device, in use, isperceived as a white light source by an observer.

Electrode layers 8 are in each case applied on the rear surfaces of thestrip-type light emitter layers 1, 2 and 3. Finally, a reflective layer5 is applied to the entire structure, and ensures that the light emittedin the rearward direction by the pn junctions of the light emitterlayers 1, 2 and 3 is reflected forward in the direction of the substrate4. The reflective layer 5 may be embodied as a metallic or elsedielectric layer. Afterward, lead wires 6 (not illustrated in FIG. 1)may be laid to each of the three OLEDs and be connected in each case tothe electrode layers 8. Via the lead wires 6, it is possible to apply tothe OLEDs different voltages from voltage sources, the common oppositepole of which is connected to the first electrode layer 7.

In the exemplary embodiment shown, each emission type is representedonly by one light emitter layer. For the case where a higher luminousintensity and/or a larger illumination area is desired, it is possibleto apply further strips of light emitter layers in an alternate order oftheir spectral type.

1. An electroluminescent light emission device suitable for use as awhite light source, comprising: a transparent substrate formed as adiffusing screen; at least one first transparent electrode layer appliedto the substrate; a plurality of electroluminescent light emitter layersarranged laterally next to one another on the at least first transparentelectrode layer and from whose emission radiation white light can begenerated by additive color mixing; at least one second electrode layerapplied to the plurality of light emitter layers; the lateral extendand/or a lateral distance of the light emitter layers amounting to atleast 100 μm; and the diffusing screen being an opal glass screen or aplastic substrate with embedded scattering centers and its thicknessbeing a multiple of the average scattering length of the radiation beamsto be mixed of the light emitter layers and being at least as large asthe distance between the light emitter layers, whereby the radiationbeams emitted by the light emitter layers are at least partly superposedon a light exit surface of the substrate.
 2. The electroluminescentlight emission device as claimed in claim 1, wherein the light emitterlayers are produced from organic materials.
 3. The electroluminescentlight emission device as claimed in claim 1 or 2, wherein a plurality offirst and/or second electrodes are arranged in such a way that lightemitter layers with a different emission spectrum can be drivenseparately from one another and those with an identical emissionspectrum can be drive jointly.
 4. The electroluminescent light emissiondevice as claimed in on of claims 1 to 2, wherein the emission spectraprovided are represented only by in each case one light emitter layer.5. The electroluminescent light emission device as claimed in one ofclaims 1 or 2, wherein the emission spectra are provided by in each casea plurality of light emitter layers.
 6. The electroluminescent lightemission device as claimed in one of claims 1 to 2, wherein the lightemitter layers are in strip form and are arranged parallel to oneanother.
 7. The electroluminescent light emission device as claimed inclaim 5, wherein the light emitter layers are arranged in an alternateorder according to their emission spectrum.
 8. The electroluminescentlight emission device as claimed in one of claims 1 to 2, wherein threetypes of light emitter layers respectively have the emission spectra ofmaxima in red, green and blue wavelength ranges.
 9. Theelectroluminescent light emission device as claimed in one of claims 1to 2, wherein two types of light emitter layers respectively have theemission spectra of maxima in yellow and blue wavelength ranges.
 10. Theelectroluminescent light emission device as claimed in one of claims 1to 2, wherein the light emitter layers are provided with a reflectivelayer on their surface remote from the substrate.
 11. Theelectroluminescent light emission device as claimed in claim 10, whereinthe second electrode layer simultaneously serves as the reflectivelayer.
 12. The electroluminescent light emission device as claimed inone of claims 1 to 2, wherein the first electrode is an ITO layerapplied to the substrate.